A supernova watched from start to finish

Something big went boom nearby. A number of telescopes watched the entire …

Nearby supernovae are rare events indeed, so the one termed SN 2006aj might have been expected to make a big splash. But the splash from this one was big indeed: four papers and a perspective in Nature (conveniently linked from a single editor's summary) and coverage on CNN. The reason for the attention all comes down to NASA's Swift observatory working as it was designed: detecting high energy gamma-rays, and rapidly spinning to observe them.

In this case, the trigger was a gamma-ray burst (named GRB060218) that signaled the star's impending collapse, and continued observations revealed a later X-ray flash, termed XRF060218. Ground based observations eventually revealed the broad-spectrum output that signaled the explosion of a star. The analysis confirms that SN 2006aj/GRB060218/XRF060218 are all one and the same, and thus these observations provide the first case where a gamma-ray burst has been definitively associated with a supernova, as well as the first time that a supernova has been imaged at the moment when the shock wave of the explosion reaches the surface of the star, bringing its power into view in the visible spectrum.

The star in question appears to have been about 40 times the mass of our sun, and belonged to a class called a Wolf-Rayet. These stars have ejected most of their lighter material, and are composed primarily of elements such as carbon and oxygen. This unusual composition and lower mass make this different from the source of the more violent Type-II supernova, and places it in a class called Type-Ic. The gamma-ray burst apparently signaled the collapse of the star's core, while the X-rays that followed apparently had two sources. One was the motion of the shockwave itself, moving at about 90 percent of the speed of light (the authors termed this "mildly relativistic"). The second appears to be generated by jets of matter being spun off by the neutron star at nearly the speed of light; this later, relativistic source kept pumping out X-rays for weeks after the shock wave had blown out the star. The visible light from the explosion also extended longer than might be expected, as the fusion reactions during stellar collapse produced a nickel isotope with a short half life. Its rapid decay started re-heating the remains after two days.

Putting all of this together, it appears that the primary thing that distinguishes the X-ray production by this Type-Ic explosion from Type-II supernovae and their gamma-ray output is simply mass. The relatively low mass of this star simply resulted in a lower energy explosion that was otherwise very similar to that produced by models of gamma-ray bursts. To a biologist like me, this suggests that current models of these explosions are on the right track, as they can explain a broad range of conditions.